This invention relates to limited motion, polarized, moving iron motors, e.g., galvanometers used in strip chart recording of biological function measurements.
There is a general requirement to obtain the maximum frequency and amplitude response possible with such motors while avoiding excessive overshoot. This requires damping the response of the motor in such a way that the motor operates as a second order system. I.e., the equation of motion of the motor is of the form, e.g., in a galvanometer, EQU I(d.sup.2 .theta./dt.sup.2)+K.theta.=F;
where I is the inertial load of the galvanometer armature and writing stylus, .theta. is the angle of rotation of the armature, K is a torsional restorative force acting to return the armature to a neutral position, and F is a torsional force applied to rotate the armature from the neutral position. F is due to a flow of current through the galvanometer drive coils where the magnitude and direction of the current determines value and direction of .theta..
One approach to controlling the galvanometer response is active velocity feedback, wherein a signal representing the angular velocity of rotation of the armature (d.theta./dt) is fed back and combined with an input signal representing the desired value of .theta.. The generally preferred method of obtaining the velocity signal, for reasons of size and complexity of construction, has been by means of a velocity coil, preferably wound on the drive coil. For this purpose the permanent magnet which is included in the glavanometer stator structure to provide a bias flux to exert the restorative torsional force on the armature as described above, also plays a role in providing the velocity signal. The bias flux is modulated by the rotation of the armature in a manner related to d.theta./dt and the velocity pickoff coil senses the variations in the bias flux to provide the velocity signal. In practice, however, the drive coil and the velocity pickoff coil are magnetically linked by a mutual inductance so that the desired velocity signal is buried in a large signal which is induced in the velocity pickoff coil by the drive current flowing in the drive coil. The coupling effects increase with the second power of the frequency of the drive current and became especially detrimental above the resonant frequency of the galvanometer system, when the induced drive currents change phase relative to the velocity signal. The common solution to this problem has been to locate the velocity pickoff coil at a substantial distance from the drive coil, but this results in increased weight, loss of structural rigidity, and increased cost and complexity. Another solution, as shown in Montagu U.S. Pat. No. 3,970,979, is to use a non-symmetric stator and drive coil structure arranged so that the velocity and drive coil magnetic circuits are separated with each including a separate portion of the armature and bias flux magnetic circuit.